1. Field of the Invention
The present invention relates to a phosphorescent material, and an organic electroluminescent device and an image display apparatus using the phosphorescent material.
2. Description of the Related Art
In an old example of an electroluminescent device, a voltage has been applied to an anthracene evaporated film to emit light (Thin Solid Films, 94, 171 (1982)). In recent years, it has been known that the area of the organic electroluminescent device among electroluminescent devices can be increased more easily than the inorganic electroluminescent device and a novel material of the organic electroluminescent device can be easily developed to provide desired color development by using the material or a combination thereof. Further, as the organic electroluminescent device has advantages including its ability to be driven at a low voltage, application research has been vigorously conducted on the transformation of an organic electroluminescent device as an electroluminescent device having high-speed responsivity and high efficiency into a device including the development of a material for the device.
For example, Macromol. Symp. 125, 1 to 48 (1997) discusses an organic electroluminescent device in which two upper and lower layers of electrodes and an organic layer including a light-emitting layer between the electrodes are formed on a transparent substrate.
Meanwhile, in recent years, investigation has been conducted on not only a conventional system utilizing fluorescence upon transition from an excited singlet exciton to a ground state but also a system utilizing phosphorescence via an excited triplet exciton. Specific examples of an organic electroluminescent device using phosphorescence is described in each of “Improved energy transfer in electrophosphorescent device” (D. F. O'Brien et al., Applied Physics Letters Vol. 74, No. 3, p. 422 (1999)) and “Very high-efficiency green organic light-emitting devices based on electrophosphorescence” (M. A. Baldo et al., Applied Physics Letters Vol. 75, No. 1, p. 4 (1999)). In each of those documents, an organic layer having a four-layer structure has been mainly used. Such a four-layer structure includes, for example, a hole-transporting layer, a light-emitting layer, an exciton diffusion-prevention layer, and an electron-transporting layer disposed in the named order from an anode side. Further, as a phosphorescent material contained in the organic electroluminescent device having such a structure, Ir(ppy)3 is widely known.
As described above, an organic electroluminescent device has recently showed significant progress. The organic electroluminescent device is characterized in that it has a high luminance at a low applied voltage, a variety of emission wavelengths, and a high-speed responsivity and can be subjected to thickness reduction and weight reduction. The characteristic suggests the potential of the organic electroluminescent device to find use in a wide variety of applications.
However in terms of practical use, at present, an optical output with a higher luminance, or a high conversion efficiency has been demanded. In addition, there still remain a large number of problems in terms of durability such as a time-dependent change due to long-term use and degradation due to an atmospheric gas containing oxygen or due to moisture. Further, good color purity and a red emission with a high efficiency are demanded when the application of the device to a full-color display or the like is taken into consideration. However, it cannot be said that those problems have been satisfactorily solved.